Discontinuous fluid pressure associated with digitally controlled causes undesirable noise emission due to the pulse-like pressure variation and due to the noise emanating from the inlet and outlet valves associated with the digital control of the fluid.
As disclosed in patent application Ser. No. DE 43 19 227.0, digitally controlled brake systems are generally known. However, it has been found in the brake system described in application number 43 19 227.0 that, on quick operation of the brake (emergency braking), an undesirable premature activation of the restrictor, connected downstream of the inlet valve, cannot be prevented with sufficient reliablity. Such a premature restrictor activation causes change of the pedal feeling and the vehicle-related gradient of pressure increase. More particularly, the reduction of the pressure-increase gradient caused by the restrictor effect reduces the braking efficiency in slip-free, dynamically stable driving operations.
European patent application 0 317 305 discloses a solenoid valve which is suitable for use in anti-lock hydraulic brake systems. The solenoid valve has a magnetic core accommodating a coil. The magnetic core is confined by a magnetic armature on one side and by a restrictor member on the other side. The restrictor member has a passage which is closable by a valve needle. The restrictor member is compressed in an axially movable fashion between the magnetic core and a housing cover in which the pressure fluid inlet is provided, so that in the operating condition, where the valve needle closes off the supply duct in the restrictor member, a differential pressure acts on either side of the restrictor member and causes displacement of the valve needle in relation to the magnetic armature. This preloads a compression spring interposed between the valve needle and the magnetic armature. The preloading force of the compression spring produced by the difference in pressure on the restrictor member causes a quick release of the magnetic armature from the magnetic core when the electromagnetic excitation is interrupted. This results in short valve opening times. A spring resetting force acting on the valve needle in the opening sense favors the quick opening of the supply duct in the restrictor member. Thus, pressure fluid propagates to an annular slot after having passed through the free passage at the valve needle. The annular slot is provided between the hollow-cylindrical inside wall of the magnetic core and the outside wall of a cylindrical part in which the valve needle extends. The fluid which emanates from the valve inlet is conducted through the supply duct in the restrictor member and the subsequent annular slot into a pressure fluid connection which leads to the pressure fluid consumer.
Therefore, an object of the present invention is to maintain a virtually modified simple structure of the brake system which resulted from the main application, and to provide a solution with respect to preventing a premature undesirable activation of the controllable restrictor.